coders.. the ultimate convert relative links to absolute links?

function absolute_url($txt, $base_url){
$needles = array('href="', 'src="', 'background="');
$new_txt = '';
if(substr($base_url,-1) != '/') $base_url .= '/';
$new_base_url = $base_url;
$base_url_parts = parse_url($base_url);

foreach($needles as $needle){
while($pos = strpos($txt, $needle)){
$pos += strlen($needle);
if(substr($txt,$pos,7) != 'http://' && substr($txt,$pos,8) != 'https://' && substr($txt,$pos,6) != 'ftp://' && substr($txt,$pos,9) != 'mailto://'){
if(substr($txt,$pos,1) == '/') $new_base_url = $base_url_parts['scheme'].'://'.$base_url_parts['host'];
$new_txt .= substr($txt,0,$pos).$new_base_url;
} else {
$new_txt .= substr($txt,0,$pos);
}
$txt = substr($txt,$pos);
}
$txt = $new_txt.$txt;
$new_txt = '';
}
return $txt;
}